Method and system of harmonic regulation

ABSTRACT

A system and method of harmonic regulation includes a harmonic regulator configured to cancel or inject harmonics into a power conversion system. A resettable integrator is provided to determine at least one harmonic coefficient of the at least one error signal harmonic. The resettable integrator determines the at least one harmonic coefficient over a single signal period and is then reset. The harmonic regulator further includes at least one adder to determine a difference of the harmonic coefficient and the reference harmonic coefficient and a regulator is provided to determine an at least one axis harmonic reference signal. The harmonic regulator outputs a three-phase final electrical reference signal that is input into a DC/AC inverter of a power conversion system.

[0001] The present application is a divisional application of U.S.patent application Ser. No. 09/681,156 filed Feb. 1, 2001, thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to harmonic regulation ofa power conversion system and, more specifically, to a method and systemof canceling and injecting harmonics into a power conversion signal.

[0003] Active filters employing active-feedback loops are increasinglybeing used to eliminate selected harmonic errors within a power signalof a power conversion or power line conditioner system. Utilizing afeedback loop and an inverter, such active filters are able to minimizethe differences between an instantaneous signal and a desired signal.The differences between the instantaneous signal and the desired signalare indicative of the distortion found or created in the signal causedby various loads in the electrical network. Oftentimes, the loads arenonlinear in nature which further accentuates the distortion in theinstantaneous signal.

[0004] Generally, nonlinear loads cause reflected harmonics to flow backinto a power source resulting in typically unwanted harmonics in theelectrical network. The active filter is used to reduce these unwantedharmonics using series and/or parallel power conversion stages. Knownactive filters reduce or cancel the harmonic effects in the powerconversion signal by implementing a harmonic regulator that applies aClarke-Park transformation techniques to convert desired feedbacksignals to a reference frame synchronous with the harmonic of interest.In these known active filters, the transformed feedback signalcontaining the unwanted harmonics may contain AC components of otherharmonics (including the fundamental) thereby requiring a low passfilter to remove unwanted frequencies in the signal. Further, theseknown active filters do not permit the displaying of the 1 harmonicmagnitudes determined by the Clarke-Park transformation without low passfiltering to eliminate AC components from the transformed signal.Additionally, known filtering systems do not allow for arbitraryreferences to control any desired harmonic to a zero or non-zero valuethereby permitting the driving of the harmonics in the feedback signalto a nonzero value.

[0005] It would therefore be desirable to have a system and methodcapable of canceling and/or injecting harmonics into a power conversionsignal. It would further be desirable to design a system and methodcapable of displaying the magnitudes of harmonic coefficients determinedby a discrete Fourier transformation and to complete the discreteFourier transformation using a resettable integrator over a singlesignal period without low pass filtering.

SUMMARY OF THE INVENTION

[0006] The present invention discloses a method and system of harmonicregulation that overcomes the aforementioned drawbacks. In one aspect ofthe present invention, a method of harmonic regulation for generating amultiphase electrical reference signal is disclosed. The method includesthe step of determining at least one reference harmonic coefficient andidentifying an energizing signal having a plurality of harmonics. Themethod further includes the step of selecting at least one harmonic ofthe energizing electrical signal and determining at least one harmoniccoefficient of the at least one harmonic of the energizing electricalsignal over one signal period. The method further includes the step ofgenerating the multiphase electrical reference signal from thepreviously determined at least one harmonic coefficient.

[0007] In accordance with another aspect of the present invention, amethod of harmonic regulation for a power conversion system isdisclosed. The method includes the steps of determining at least onereference harmonic coefficient and receiving at least one referenceinput electrical signal and at least one feedback electrical signal. Anelectrical error signal is then determined from the at least onereference input electrical signal and the at least one feedbackelectrical signal. Next, the method selects at least one harmonic havingat least one coefficient from the electrical error signal anddetermining an electrical angle, Be. The method then determines a sinesignal and a cosine signal of the selected at least one harmonic at theknown inverter electrical angle and integrates the sine signal and thecosine signal over one signal time period, T. The signal time period Tis equivalent to the reciprocal of the inverter base frequency. Theintegrated sine signal and the integrated cosine signal are thencompared to the at least one reference harmonic coefficient and at leastone harmonic axis value is determined therefrom. The method furtherincludes the step of injecting the at least one harmonic axis signalinto a power conversion system.

[0008] In yet another aspect of the present invention, a harmonicregulator is disclosed. The harmonic regulator includes a feedbacksignal detector configured to determine a feedback error signal andfurther configured to determine an electrical angle corresponding to aselected harmonic and further configured to determine a feedback signalperiod. The harmonic regulator further includes a harmonic selectorconfigured to select a feedback error signal harmonic. A resettableintegrator configured to determine at least one harmonic coefficient ofthe at least one error signal harmonic is further provided wherein theat least one harmonic coefficient is determined over a single feedbacksignal period. The harmonic regulator further includes at least oneadder configured to determine a difference of the at least one harmoniccoefficient and the at least one reference harmonic coefficient. Aregulator is provided and configured to amplify and integrate thedifference and is further configured to determine at least one harmonicaxis reference signal. An inverse rotator is provided to receive the atleast one harmonic axis reference signal and to generate an inverserotator output. The harmonic regulator further includes at least onesummer configured to determine a final electrical reference signal fromthe inverse rotator output and an inverter current reference output.

[0009] In yet a further aspect of the present invention, a powerconversion system is provided. The system includes a number ofelectromagnetic interference (EMI) filters, an AC/DC converter, a DClink filter, a DC/AC inverter, an AC filter, a plurality of feedbacksensors including a plurality of voltage feedback sensors and 4 currentfeedback sensors, and a feedback loop that includes a number of voltageand current feedback sensor conditioners. The power conversion systemfurther includes an inverter control having a harmonic regulatorincluding a resettable integrator wherein the harmonic regulator isconfigured to determine at least one reference harmonic coefficient andidentify a distortion signal having a plurality of harmonics. Theharmonic regulator of the power conversion system is further configuredto select at least one harmonic of the distortion signal and determineat least one harmonic coefficient therefrom over a single signal period.The harmonic regulator is further configured to generate a multiphaseelectrical reference signal for the power conversion system.

[0010] In an another aspect of the present invention, a computer programcomprising a set of instructions to cause one or more computers toinject at least one harmonic axis signal into a power conversion system.The computer program further causes the one or more computers todetermine at least one reference harmonic coefficient and further causesthe one or more computers to receive at least one reference electricalsignal and at least one instantaneous electrical signal. The set ofinstructions of the computer program further causes the one or morecomputers to determine an electrical error signal from the at least onereference electrical signal and the at least one instantaneouselectrical signal. The computer program further cause the one or morecomputers to select at least one harmonic having at least onecoefficient from the electrical error signal and to apply a discreteFourier transformation to the at least one harmonic selected todetermine a first integrated signal and a second integrated signal. Theone or more computers are then caused to compare the first integratedsignal and the second integrated signal to the at least one referenceharmonic coefficient and to determine at least one harmonic axis signaltherefrom. The computer program then causes the one or more computers toinject the at least one harmonic axis signal into the power conversionsystem.

[0011] Various other features, objects and advantages of the presentinvention will be made apparent from the following detailed descriptionand the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The drawings illustrate one preferred embodiment presentlycontemplated for carrying out the invention.

[0013] In the drawings:

[0014]FIG. 1 is a schematic block diagram of a power conversion systemin accordance with the present invention;

[0015]FIG. 2 is a schematic block diagram of the inverter control shownin FIG. 1;

[0016]FIG. 3 is a schematic block diagram of the harmonic regulator inaccordance with the present invention shown in FIG. 2;

[0017]FIG. 4A is a graph illustrating a simulated inverter outputvoltage spectrum without harmonic regulation; and

[0018]FIG. 4B is a graph in accordance with the present inventionillustrating a simulated inverter output spectrum with harmonicregulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] Referring to FIG. 1, a power conversion system 10 is disclosed.The system 10 includes an electromagnetic interference filter 12 thatreceives, in a preferred embodiment, a three-phase input 11 of 460 VACat 60 Hz. The EMI filter 12 reduces or filters out any interference inthe input signal 11. After filtering, the input signal 11 is input intoan AC/DC converter 14. Implementing a known method, the AC input signalis converted to a DC signal and is input into a DC link filter 16. TheDC link filter 16 blocks out any unwanted AC components in the convertedsignal using, in a preferred embodiment, at least one capacitor (notshown). A DC/AC inverter 18 receives the filtered output from the DClink filter 16. In a preferred embodiment the DC/AC inverter 18 includesa number of Insulated Gate Bi-Polar Transistors (IGBT). IBGT's havesuperior on-state characteristics, reasonable switching speed, andexcellent safe operating area, therefore, in a preferred embodiment, areused to invert the signal. Output from the DC/AC inverter 18 is then fedto an AC filter 20 that removes or filters out any unwanted portions ofthe inverted signal. Output from AC filter 20 is then fed to anotherelectromagnetic interference filter 22 for removing or reducing anyelectromagnetic interference remaining in the signal. The powerconversion system 10 further includes a feedback loop 24 that includes aplurality of voltage and current feedback sensors 26 and an invertercontrol system 28, that will be discussed in greater detail withreference to FIG. 2. Ultimately, an output signal 30 is generatedgenerally characterized as a three-phase variable voltage, variablefrequency output signal that is fed to a three-phase load (not shown).

[0020] A schematic diagram of the inverter control 28 is shown in detailin FIG. 2. The voltage and current feedback sensors 26 of FIG. 1 detecta three-phase voltage signal 32, FIG. 2, and a three-phase currentsignal 34. A three-phase to two-phase transformation controller 36performs a forward Clarke transform on the rotating three-phase voltagesignal 32 and the three-phase current signal 34. The two-phase voltagefundamental components, V_(qs) and V_(ds), and the two-phase currentfundamental components, I_(qs) and I_(ds), are input into a stationaryto synchronous transformation controller 38. Implementing a well knowntransformation method often referred to as a reverse Park transform, asynchronous quadrature voltage component 40, a synchronous directvoltage component 42, a synchronous quadrature current component 47, anda synchronous direct current component 46 are generated. A differencejunction 45, such as an adder, receives the voltage component 40 andcompares the voltage component 40 to a reference synchronous quadraturevoltage component 44. A Proportional Integral (PI) regulator consistingof gain block 49 and integrator 48 receives the difference junctionoutput 43 and each of the gain block 46 and integrator 48 of the PIregulator generates an output signal that is fed to summing junction 50,such as an adder, that sums the outputs of the PI regulator consistingof gain block 49 and integrator 48. A summing junction output signal 52is fed to a current limit function controller 54.

[0021] Referring to voltage component 42, an amplifier 56 receivesvoltage component 42 and, in a preferred embodiment, amplifies thevoltage component 42 by −1 to regulate the d-axis voltage to zero. Thegain block 56 generates an output signal 57 that is an input to a Piregulator comprising gain block 58 and integrator 60 which generatesoutput signals that are summed by an adder 62. The current limitfunction controller 54 receives summer output signal 64 as well assynchronous quadrature current component 47 and synchronous directcurrent component 46. Using current reference signals 52 and 64 andcurrent feedback signals 47 and 46, the current limit functioncontroller 54 generates a synchronous quadrature reference currentsignal 66 and a synchronous direct reference current signal 68 which arereceived as inputs by a synchronous to stationary transformationcontroller 70, often referred to as a reverse Park transform.

[0022] Still referring to FIG. 2, using the electrical angle, θ_(e), thesynchronous stationary transformation controller or reverse Parktransform 70 determines a stationary quadrature reference current signal72 and a stationary direct reference current signal 74 that are input toa two-phase to three-phase transformation controller 76, often referredto as a reverse Clarke transform. Controller 76 outputs a threephaserotating reference current signal 78 wherein each reference phase isadded to a complementary phase signal by an adder or summer 80. Aharmonic regulator 82 generates the three-phase complementary currentsignal using a single phase voltage input, the synchronous quadraturereference voltage signal 44, and the electrical angle, θ_(e). Theharmonic regulator 82 will be discussed with greater detail withparticular reference to FIG. 3. A hysteresis current control 84 receivesthe output signals from the summers 80 and further receives inverterpole current feedback signals 86 and outputs a plurality of IGBT gatedrive signals 88 which are received by the DC/AC inverter 18 of FIG. 1.

[0023] Referring to FIG. 3, a schematic diagram of the harmonicregulator 82 is shown in accordance with the present invention. Theharmonic regulator 82 includes a feedback signal detector 90 thatreceives the synchronous quadrature reference voltage signal 44, onephase of the voltage signal 32 a, and the electrical angle, θ_(e),corresponding to a particular signal frequency and determines a feedbackerror signal 10 indicative of the distortion in the input signals 44 and32 a. In one embodiment, the feedback signal detector determines theerror signal using the following equation:

Vab−{square root}3·Vqe(ref)·sin(θe−2π/3)  Eqn. 1

[0024] One of ordinary skill in the art will appreciate however, thatalternative functions may be used to calculate the distortion of theinput signals 44 and 32 a. The feedback error signal 92 then undergoes adiscrete Fourier transformation 94.

[0025] The distortion signal 92 after being modified by a scaler value96 is received by at least one signal controller 98. The signalcontroller 98 using the electrical angle, Be, at the harmonic selectedby the harmonic selector (not shown) determines a signal indicative ofthe cosine 100 of the distortion signal 92 and a signal indicative ofthe sine 102 of the distortion signal 92. A resettable integrator 104receives signals 100 and 102, and integrates the signals 100, 102 over asingle signal period T. It should be noted, however, that the electricalangle, θ_(e), is inverted to a value of −θ_(e) for any 6n−1 harmonic,where n equals any positive real integer.

[0026] Still referring to FIG. 3, since the output signals of theintegrator are determined over a single period, the presence of any ACcomponents in the signal are eliminated yielding a pure DC value equalto the d and q-axis components of the specified harmonic which allowsfor displaying of the integrator output signals 106, 108 visually to auser, such as with a computer monitor (not shown). A difference junction110 receives the integrator output signals 106, 108 and compares each toa reference harmonic coefficient 112, 114. The reference harmoniccoefficients 112 and 114 are assigned arbitrary values depending uponthe goal of the harmonic regulator. For example, to cancel a harmonic ofthe power conversion signal each reference harmonic coefficient is setto an initial value of zero. Conversely, however, 11 to inject harmonicsinto a power conversion system reference harmonic coefficients 112 and114 are initialized to some real value. The harmonic regulator 82 willthen generate a signal indicative of the harmonic reference coefficientas initialized.

[0027] Adders 110 output a harmonic error signal 116 which is receivedby integrators 118. Integrators 118 integrate the harmonic error signals116 with a gain of K, wherein K is an arbitrary value selected dependingupon the particular use or function of the harmonic regulator 82.Integrators 118 integrate the harmonic error 116 until harmonic errorsignal 116 is driven to zero. In a preferred embodiment, signals 120 and122 are current signals. The synchronous to stationary transformation(reverse Park transform) controller 124 receives signals 120 and 122 aswell as the electrical angle, θ_(e), corresponding to the harmonicselected by the harmonic selector. The synchronous to stationarytransformation 124 determines stationary reference frame signals for thed-axis and q-axis harmonic reference signals using the well knownreverse Park transformation technique. The stationary harmonic referencecurrent signals are received by a two-phase to three-phasetransformation controller or reverse Clarke transformation controller126 which determines the three-phase complementary current signals whichare summed with reference current signals 78 as was discussed inreference to FIG. 2.

[0028] The harmonic regulator 82, as shown in FIG. 3, contemplatesharmonic regulation, i.e., cancellation or injection, of a myriad ofdistortion signal harmonics. Therefore, to simultaneously inject orcancel a number of harmonics of a signal, the present inventioncontemplates a redundancy of the harmonic regulator components discussedin the paragraphs above. The redundancy of components is shown in FIG. 3with a second set of components having like numbers as those discussedabove, but including an “a” in referencing. A third set of components islikewise shown using an “x” reference with like reference numerals. Thefunction of each is similar to that previously discussed.

[0029] Accordingly, the present invention discloses a method of harmonicregulation. The method includes the steps of determining at least onereference harmonic coefficient that will be set to an initial value ofzero to cancel harmonics of an energizing electrical signal or to anynon-zero value to inject harmonics into an energizing electrical signalof a power conversion system. The method further includes the step ofselecting at least one harmonic of the energizing electrical signal anddetermining at least one harmonic coefficient of the at least oneharmonic of the energizing electrical signal over one signal periodusing a discrete Fourier transformation. The step of performing adiscrete Fourier transformation further includes determining a signalindicative of the sine of the energizing electrical signal and a signalindicative of the cosine of the energizing electrical signal. Both thesine and cosine signals are determined using the electrical anglecorresponding to the selected harmonic of the energizing electricalsignal. The method further includes the step of integrating the cosineand sine signals over one signal period using a resettable integrator.The integration of the cosine signal results in an “a” harmonic and theintegration of the sine signal results in a “b” harmonic signal. Next,the “a” and “b” harmonic coefficients are subtracted from an “a”reference harmonic coefficient and a “b” reference harmonic coefficientto produce two error signals. The error signals are then integrated.Following the integration step, the reference signals undergo a reverseClarke and Park transformation to generate a three-phase harmonicreference signal. The three-phase harmonic reference signal is thenadded to additional three-phase harmonic reference signals for eachremaining harmonic thereby forming a final complementary referencecurrent signal.

[0030] To implement the aforementioned method, the present inventionfurther includes a computer program comprising a set of instructions forone or more computers to cause the one or more computers to cancel orinject harmonics into a power conversion system signal. The set ofinstructions further cause the one or more computers to determine atleast one reference harmonic coefficient and receive at least onereference electrical signal and at least one instantaneous electricalsignal. The set of instructions further cause the one or more computersto determine an electrical error signal from the at least one referenceelectrical signal and the at least one instantaneous electrical signal.The electrical error signal is indicative of distortion in the powerconversion system. The set of instructions further cause the one or morecomputers to select at least one harmonic having at least onecoefficient and in a preferred embodiment an “a” coefficient and a “b”coefficient from the electrical error signal. The set of instructionsfurther cause the one or more computers to apply a discrete Fouriertransformation to the at least one harmonic to determine a firstintegrated and a second integrated signal.

[0031] In a preferred embodiment the first integrated signal isindicative of the sine of the electrical error signal and the secondintegrated signal is indicative of the cosine of the electrical errorsignal. To determine the sine and cosine of the electrical error signal,the one or more computers determine an electrical angle, θ_(e), for aselected harmonic and implement that angle during the discrete Fouriertransformation. In a preferred embodiment the magnitude of the firstintegrated and the second integrated signals are visually displayed on amonitor or other visual means to a user or technician. Moreover, thefirst integrated signal and the second integrated signal are determinedover a single signal period, T. Therefore, low pass filtering of theintegrated signals is unnecessary.

[0032] After applying the discrete Fourier transformation, the set ofinstructions further cause the one or more computers to compare thefirst integrated signal and the second integrated signal to at least onereference harmonic coefficient. The at least one reference harmoniccoefficient may have any arbitrary value depending upon the intendedgoal of the harmonic regulator. For example, to cancel the selectedharmonic of a power conversion signal the at least one referenceharmonic coefficient for both the “a” reference harmonic coefficient andthe “b” reference harmonic coefficient should be initialized to zero.Conversely, to inject the selected harmonic into the power conversionsignal having an x value, the at least one reference harmoniccoefficients are initialized with that x value. Next, the set ofinstructions cause the one or more computers, in a preferred embodiment,to subtract the first integrated signal from the at least one referenceharmonic coefficient and subtract the second integrated signal from theat least one reference harmonic coefficient. Resulting therefrom is anadjustment signal that is integrated with a gain of K to determine adirect synchronous harmonic reference signal and a quadraturesynchronous harmonic reference signal.

[0033] The set of instructions then cause the one or more computers toperform a synchronous to stationary transformation on the directsynchronous harmonic reference signal and the quadrature synchronousharmonic reference signal. This transformation is well known andgenerally referred to as a forward Park transformation. After the d-axisstationary harmonic reference signal and the qaxis stationary harmonicreference signal are determined, the set of instructions further causethe one or more computers to perform a two-phase to three-phasetransformation. This transformation is well known and generally referredto as a reverse Clarke transformation. The three-phase output of thereverse Clarke transformation is then added to any additionalthree-phase harmonic signals for other selected harmonics to generate afinal three-phase complementary reference signal.

[0034] The method of harmonic regulation as well as the actsaccomplished by the one or more computers when instructed by the set ofinstructions regulate the harmonics of an error signal indicative ofdistortion in the energizing electrical signal over a single signalperiod, T. The method/set of instructions repeat continuously in a loopuntil the coefficients of the selected harmonic of the distortion orerror signal match the “a” and “b” reference coefficients. Referring nowto FIGS. 4A and 4B, a specific example based on computer simulation willbe discussed. To cancel the fifth and seventh harmonics of an energizingelectrical signal the harmonic reference coefficients are initialized toa value of zero. To drive the fifth and seventh harmonics of thedistortion signal to zero, however, may require several loops of theenergizing electrical signal through the harmonic regulator. The numberof loops depends upon the initial value of the harmonic coefficients ofthe distortion of the energizing electrical signal. An initialenergizing electrical signal 150 a with the fifth and seventh harmonicspresent 155 a, 157 a is shown in FIG. 4A. FIG. 4B, however, illustratesthe signal 150 b of FIG. 4A with the fifth and seventh harmonics 155 b,157 b cancelled in accordance with the present invention. Conversely,the present invention contemplates the injection of harmonics into asignal wherein the signal 150 b, FIG. 4B is the initial signal andsignal 150A, FIG. 4A, with the fifth and seventh harmonics 155 a, 157 apresent is the final signal generated by the harmonic regulator inaccordance with the present invention.

[0035] The present invention has been described in terms of thepreferred embodiment, and it is recognized that equivalents,alternatives, and modifications, aside from those expressly stated, arepossible and within the scope of the appending claims.

What is claimed is:
 1. A power conversion system, comprising: a number of EMI filters; an AC/DC converter; a DC link filter; a DC/AC inverter; an AC filter; a feedback loop including a number of voltage feedback sensors and current feedback sensors; and an inverter control including an harmonic regulator having a resettable integrator wherein the harmonic regulator is configured to: determine at least one reference harmonic coefficient; identify a distortion signal having a plurality of harmonics; select at least one harmonic of the distortion signal; determine at least one harmonic coefficient of the at least one harmonic of the distortion signal over one signal period; and generate a multiphase electrical reference signal.
 2. The power conversion system of claim 1 wherein the resettable integrator is reset to an initial value after the at least one reference harmonic is determined.
 3. The power conversion system of claim 1 wherein the harmonic regulator is further configured to determine an error factor, wherein the error factor includes a difference of the at least one reference harmonic coefficient and the at least one harmonic coefficient.
 4. The power conversion system of claim 1 wherein the harmonic regulator is further configured to multiply an amplification factor to the error factor and determine at least one axis harmonic reference value therefrom.
 5. The power conversion system of claim 4 wherein the harmonic regulator is further configured to determine the at least one axis harmonic reference value by integrating the product of the error factor and the amplification factor.
 6. The power conversion system of claim 5 wherein the harmonic regulator is further configured to inversely rotate the at least one axis harmonic reference at the electrical angle, θ_(e), to determine the multiphase electrical reference signal.
 7. The power conversion system of claim 1 wherein the harmonic regulator is further configured to generate the multiphase electrical reference signal without low pass filtering.
 8. The power conversion system of claim 1 wherein the harmonic regulator is further configured to display the at least one harmonic coefficient 